To revert to Google as your preferred search engine, please do the following.

• In the location bar, type about:config and hit Enter.
  
  
• In the filter at the top, type: keyword.URL
  
  
• Double click it and remove whatever's in there and replace it with http://www.google.com/search?q=
  
  

The URL to add in "keyword.URL" becomes a link in this post, so right click it and choose "Copy Link Location" to copy it to the Windows clipboard. Then hit CTRL+V to paste it. Saves you having to type the whole thing.

To reset your home page, do the following.

• Go to the site you want to set as your homepage.
  
  
• Click the orange Firefox button and go to Options | Options | General.
  
  
• Make sure it says "Show My Homepage" in the first dropdown menu.
  
  
• Click the button called "Use Current Pages" to set the homepage to the one you have on the screen.
  
  

After you complete the above steps, install this add-on to prevent another search engine from modifying your preferences: https://addons.mozilla.org/en-US/firefox/addon/browserprotect/

thank you Xircal, I tried doing this but it didn't work :( after restart my home page reset to "QOOQLLE.com" and my proxy also reset to "No Proxy" (BTW I usually use system Proxy setting. BTW I scanned my PC many Times With Kaspersky internet security and Trojan Remover tool but each time the same result "My PC is Clean"

Could you please download the free version of Malwarebytes from http://www.malwarebytes.org/products/malwarebytes_free and run a full system scan to eliminate malware as the culprit.

Thank you very much Xircal ... it solved the problem.

You're welcome.